1. Field of the Invention
This invention relates to a thermo-shrinkable film that is suitable for use as a wrapping material to be used as a covering, to hold objects together, as an outer wrapping, etc. In particular, it relates to a polyester-type film (here and below, the word "film" also includes the meaning "sheet") with thermoshrinkable properties that is suitable for use as a wrapping material.
2. Description of the Prior Art
Thermo-shrinkable plastic films are used as coverings, to hold objects together, and as an outer wrapping for bottles (including both glass and plastic bottles), cans, and other kinds of containers, and for cylindrical objects (pipes, rods, wood, and other kinds of cylindrical objects). For example, such films are used for covering the cap, shoulder, or bulge of bottles, or for the entire bottle, for the purpose of labelling, protection, parceling, or increasing the value of the product, and for other reasons. In addition, such films may be used as a covering to package such objects as boxes, bottles, boards, rods, or notebooks together in groups, and such films may also be attached closely as a wrapping. The uses mentioned above take advantage of the shrinkability and the shrinking stress of the film.
As the material for the film mentioned above, polyvinylchloride, polystyrene, polyethylene, hydrochlorinated rubber, etc. can be used. Usually, these films are made into a tube shape, and after being wrapped around a bottle or around more than one pipe, for example, the film is shrunk by the application of heat so as to wrap or parcel together the object or objects.
However, the conventional thermo-shrinkable plastic films described above have the following disadvantages.
(a) One-axis shrinkability
When the thermo-shrinkable films mentioned above are, for example, wrapped around a bottle and shrunk around the circumference of the bottle so as to make a label, it is desirable that the films can shrink greatly in one specific direction (around the bottle), and not shrink in the direction at right angles to this direction (that is, the long axis of the bottle). To increase the shrinkability of a film like this in one direction only, it is effective to stretch the film so as to orient the polymer molecules that form the film in the said direction. However, when the polymer molecules are oriented, the film readily splits, and in addition, because fibrillation has become easier, the strength of the film decreases. In particular, if the bottle mentioned above is dropped, the strength of the film in the direction of the long axis of the bottle is important for preventing breakage of the bottle, and therefore an oriented polymer film obtained by stretching in only one direction is not necessarily appropriate. In other uses, the property of conferring resistance to shock is often of importance, as well.
(b) Resistance to heat
The conventional films mentioned above cannot withstand either boiling or retorting, so they cannot undergo sterilization procedures at high temperatures. For example, if the conventional films mentioned above are retorted at high temperatures, they rupture during treatment.
(c) Printability
If, for example, polyvinylchloride film is printed by the half-tone process, its compatibility with the ink is poor. Also, because the heat-resistance of polyvinyl chloride is poor, polymer gel is likely to be formed in the film when the film is being made. Therefore, ink-pinholes form readily during printing because of these polymer gel. For that reason, during a continuous tube-formation and printing process, ink-pinholes develop in the long film. If this film is supplied to an automatic labeling machine, the product is formed with the pinholes still in existence, which requires a final inspection of all products, resulting in a decrease of productivity. If pinholes are eliminated by inspection after printing is finished, then, after the area with pinholes is cut out, the remaining portions are joined with adhesive tape, so that the film will once more be continuous. For this reason, products containing seams are made, so a step in which these unacceptable products are identified and removed becomes necessary.
With the conventional films mentioned above, there is shrinkage with the passage of time after manufacture, and this shrinkage causes a decrease in the pitch of printing, so that it is not possible to print with high accuracy. This shrinkage with time proceeds under a temperature at which the films are delivered. Therefore, shrinkable film made of polyvinyl chloride, etc., for example, must be stored in a low-temperature warehouse, or delivered by refrigerator car.
(d) Crazing
Crazing occurs readily with polyethylene film. Therefore, for example, if a chemical agent comes in contact with the film during use, the film is readily damaged. Crazing also gives rise to a rough surface for printing.
(e) Industrial waste pollution
When, for example, polyvinylchloride film is burned, hydrogen chloride gas is produced, giving rise to the problem of corrosion of the incinerator and of environmental pollution.
(f) Shrinkage spots
With the conventional films mentioned above, shrinkage in response to heat is often not uniform. When heating gives rise to some areas that are satisfactorily shrunken and also to areas with insufficient shrinkage, the surface of the film forms irregular protrusions and indentations, and does not fit to the object that it encloses. This tendency is particularly strong in high-speed wrapping, in which the heat-shrinkage occurs in a short period of time. If the film is printed with metallic ink, there is the demerit that conspicuous colored spots will form because of the uneven shrinkage.